Channel Quality Reporting
Channel quality information is used in a multi-user communication system, such as for example 3GPP LTE (Long Term Evolution) to determine the quality of channel resource(s) for one or more users. This information may be used to aid in a multi-user scheduling algorithm to assign channel resources to different users, or to adapt link parameters such as modulation scheme, coding rate or transmit power, so as to exploit the assigned channel resource to its fullest potential.
A channel resource may be defined as a “resource block” as exemplarily illustrated in FIG. 1 where a multi-carrier communication system, e.g. employing OFDM as for example discussed in the LTE work item of 3GPP, is assumed. More generally, it may be assumed that a resource block designates the smallest resource unit on an air interface of a mobile communication that can be assigned by a scheduler. The dimensions of a resource block may be any combination of time (e.g. time slot, sub-frame, frame, etc. for time division multiplex (TDM)), frequency (e.g. subband, carrier frequency, etc. for frequency division multiplex (FDM)), code (e.g. spreading code for code division multiplex (CDM)), antenna (e.g. Multiple Input Multiple Output (MIMO)), etc. depending on the access scheme used in the mobile communication system.
Assuming that the smallest resource unit is a resource block, in the ideal case channel quality information for all resource blocks and all users should be always available. However, due to constrained capacity of the feedback channel this is most likely not feasible or even impossible. Therefore reduction or compression techniques are required so as to reduce the channel quality feedback signalling overhead, e.g. by transmitting channel quality information only for a subset of resource blocks for a given user.
In 3GPP LTE, the smallest unit for which channel quality is reported is called a subband, which consists of multiple frequency-adjacent resource blocks.
Channel Quality Feedback Elements
Commonly, mobile communication systems define special control signalling that is used to convey the channel quality feedback. In 3GPP LTE, there exist three basic elements which may or may not be given as feedback for the channel quality. These channel quality elements are:                MCSI: Modulation and Coding Scheme Indicator, sometimes referred to as Channel Quality Indicator (CQI) in the LTE specification        PMI: Precoding Matrix Indicator        RI: Rank Indicator        
The MCSI suggests a modulation and coding scheme that should be employed for transmission, while the PMI points to a pre-coding matrix/vector that is to be employed for multi-antenna transmission (MIMO) using a transmission matrix rank that is given by the RI. Details about the involved reporting and transmission mechanisms are given in the following specifications to which it is referred for further reading (all documents available at http://www.3gpp.org and incorporated herein by reference):                3GPP TS 36.211, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels and modulation”, version 8.5.0, particularly sections 6.3.3, 6.3.4,        3GPP TS 36.212, “Evolved Universal Terrestrial Radio Access (E-UTRA); Multiplexing and channel coding”, version 8.5.0, particularly sections 5.2.2, 5.2.4, 5.3.3,        
3GPP TS 36.213, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures”, version 8.5.0, particularly sections 7.1.7, and 7.2.
In 3GPP LTE, not all of the above identified three channel quality elements are reported at any time. The elements being actually reported depends mainly on the configured reporting mode. It should be noted that 3GPP LTE also supports the transmission of two codeword (i.e. two codeword of user data (transport blocks) may be multiplexed to and transmitted in a single sub-frame), so that feedback may be given either for one or two codewords. Some details are provided in the next sections and in Table 1 below. It should be noted that this information is based on 3GPP TS 36.213, section 7.2.1 mentioned above.
TABLE 1TransmissionModeModeModeModeModeMode StateMode 1-22-03-02-23-1Single-antennaNANA2430NANAport 0Transmit2TX or 4TXNA2430NANAdiversityantennasClosed-loop2TX antennas30NANA2832spatialRI = 1multiplexing2TX antennas213261RI > 14TX antennas563234RI = 14TX antennas603864RI > 1Open-loop2TX antennasNA2430NANAspatial4TX antennas2430multiplexingMulti-user2TX antennasNANANANA32MIMO4TX antennas34Closed-loop2TX antennas30NANA2832rank-14TX antennas563234precoding
The individual reporting modes for the channel quality feedback is currently defined in 3GPP LTE as follows:
Reporting Mode 1-2
Contents of this report:                One wideband MCSI value per codeword        One preferred PMI for each subband        In case of transmission modes other than transmission mode 4: One RI valueReporting Mode 2-0        
Contents of this report:                One wideband MCSI value        Positions of M selected subbands        One MCSI value for M selected subbands (2 bits differential to wideband MCSI value, non-negative)        In case of transmission modes other than transmission mode 3: One RI valueReporting Mode 2-2        
Contents of this report:                One wideband MCSI value per codeword        One preferred PMI for wideband        Positions of M selected subbands        One MCSI value for M selected subbands per codeword (2 bits differential to wideband MCSI value, non-negative)        One preferred PMI for M selected subbands        In case of transmission modes other than transmission mode 4: One RI value        
For transmission mode 4 the reported PMI and MCSI values are calculated conditioned on the reported RI. For other transmission modes they are reported conditioned on rank 1.
Reporting Mode 3-0
                Contents of this report:        One wideband MCSI value        One MCSI value per subband (2 bits differential to wideband MCSI value)        In case of transmission modes other than transmission mode 3: One RI valueReporting Mode 3-1        Contents of this report:        One wideband MCSI value per codeword        One preferred PMI for wideband        One MCSI value per codeword per subband (2 bits differential to wideband MCSI value)        In case of transmission modes other than transmission mode 4: One RI value        
It should be noted that the term subband is here used so as to represent a number of resource blocks as outlined earlier, while the term wideband represents the whole set of resource blocks in a set of subbands as generally pre-defined by signalling. In the context of 3GPP LTE and LTE-A, the wideband always represents the whole cell bandwidth, i.e. a frequency range of up to 20 MHz.
In the 3GPP LTE downlink, OFDM is employed. Data may be transmitted utilizing a frequency bandwidth of up to 20 MHz in a single cell. For the enhancements presently planned for 3GPP LTE (also referred to as LTE-A, where A stands for “advanced”) also OFDM is used in the downlink further using so-called “LTE carrier aggregation” to support frequency bandwidths of up to 100 MHz in a single cell. Each such 3GPP LTE carrier is then commonly referred to as a component carrier (CoCa). A frequency bandwidth up to 100 MHz will be most likely achieved by using five 3GPP LTE carriers (component carriers) in parallel, each of a bandwidth of 20 MHz.
In 3GPP LTE, a simple mechanism is foreseen to trigger the so-called aperiodic channel quality feedback from a user equipment. A Node B in the radio access network send a L1/L2 control signal to the user equipment to request the transmission of the so-called aperiodic CQI report (see 3GPP TS 36.212, section 5.3.3.1.1 and 3GPP TS 36.213, section 7.2.1 for details). Another possibility to trigger the provision of aperiodic channel quality feedback by the user equipments is linked to the random access procedure (see 3GPP TS 36.213, section 6.2). Furthermore, a trigger may also be implemented by an activation or configuration of a periodic CQI report (see 3GPP TS 36.213, section 7.2.2).
Whenever a trigger for providing channel quality feedback is received by the user equipment, the user equipment subsequently transmits the channel quality feedback to the Node B. Commonly, the channel quality feedback (i.e. the CQI report) is multiplexed with uplink (user) data on the Physical Uplink Shared CHannel (PUSCH) resources that have been assigned to the user equipment by L1/L2 signalling by the scheduler (Node B).
Since the channel quality feedback is multiplexed with data on the PUSCH, care must be taken that the user equipment and the Node B have the same understanding about which part of an uplink transmission on the PUSCH within a sub-frame is the channel quality feedback and which part is the user data. In 3GPP LTE, this is usually not an issue because the Node B configures the reporting mode and sets the aperiodic CQI trigger, so it knows when the UE transmits the feedback and also knows the size of the channel quality feedback and—by specification—the location of the feedback and data part within the sub-frame so that the individual parts may be recovered.
In case of a multi-cell operation (e.g. during soft-handover), a user equipment may actually receive the L1/L2 control channel(s) from multiple Node Bs. Generally each of these Node Bs may ask for channel quality feedback individually, without knowledge about other Node Bs feedback requests. This alone may cause different understandings between user equipment and one or more Node Bs on the contents of an uplink transmission. For example, in case the user equipment is communicating the same data to two Node Bs on the uplink, and assuming that one of the Node Bs requests a channel quality report from the user equipment, the other Node B may not be aware of the user equipment multiplexing channel quality feedback and user data in a sub-frame and erroneously assumes that the sub-frame contains user data.
In addition, Discontinuous Reception (DRX) and miss of trigger events can further complicate the situation (see below).
Also for the future enhancement foreseen in 3GPP LTE-A, there are numerous issues why a common understanding of the content of a sub-frame may be disturbed, which leads to inefficient or erroneous transmission of the channel quality feedback and the data part. The general problem is that there could be a diverging understanding for how many of the component carriers the channel quality feedback is requested/transmitted. This circumstance is illustrated in FIG. 2, where transmitter and receiver may have a different understanding of the border between CQI part and Data part of a transmission. This in turn may lead to corruption of the whole CQI and/or data transmission, because data bits may be interpreted as if they were CQI bits, thus corrupting the CQI message. On the other hand, situation may occur where CQI bits are assumed to be data bits, thus corrupting the data transmission. The least possible impact would be that FEC redundancy for CQI and/or data is missing, which reduces the error resilience of the involved FEC.
Furthermore, another situation that may lead to a diverging understanding of the content of a transmission in 3GPP LTE is resulting from the user equipments being able to go into a DRX mode when it is not receiving any control signal for a defined number of sub-frames. Since the Node B may send a control signal, but due to erroneous reception (e.g. as a result of noise on the channel, etc.) the user equipment is not aware of the control signal, so that it may enter DRX mode even though the Node B believes the user equipment to still be in an active mode.
For 3GPP LTE-A where LTE carrier aggregation will be most likely used, it is possible that the user equipment goes into DRX mode per LTE carrier (component carrier). In other words, the UE can be in DRX mode for some component carriers available for communication, while it is still active for other component carriers. This situation is exemplarily illustrated in FIG. 3. Since the user equipment is not receiving or processing any signals from component carriers for which the user equipment is in DRX mode, it can be assumed that no CQI can be measured and reported for such component carriers.
The problem for CQI reporting is that the user equipment can only report channel quality elements for component carriers for which it is not in DRX mode. Therefore if the understanding on DRX mode for the component carriers is different between user equipment and Node B, user equipment and Node B will have different understandings on the content of the channel quality feedback. This is exemplarily highlighted in FIG. 4.
Furthermore, it may be also assumed that in 3GPP LTE-A CQI reports may also be triggered in a similar fashion as in 3GPP LTE. Even though the exact triggering mechanism for 3GPP LTE-A is not clear yet, it is possible that aperiodic CQI reporting is called for a given component carrier only, if the aperiodic CQI trigger bit in the L1/L2 control channel within the same component carrier is activated. This is illustrated in FIG. 5. Consequently, due to noise etc. it can occur that the Node B actually transmits CQI feedback triggers for three component carriers, but the user equipment is aware of only two of them. Accordingly, the user equipment would send a CQI report for only two component carriers, but the Node B assumes that the CQI report contains channel quality feedback for three component carriers. This scenario is exemplarily illustrated in FIG. 6.
Another situation, where a (detailed) reporting on the channel quality of a component carrier may be not desirable in view of transmission efficiency, is a situation where the channel conditions of the component carrier indicate a very low signal-to-noise ratio. In case the channel condition is actually very bad for a user equipment, it may not be required or reasonable to transmit a large feedback report to the Node B, since the channel quality feedback is mainly designed to be used to determine the resources and transmission parameters for data to a user equipment. If channel quality does not allow for data to be transmitted on the component carrier, the Node B will not schedule (allocate) any resources to the user equipment but instead wait (in time) until the channel conditions improve or the Node B assigns the user equipment to other frequency ranges or even component carriers where the channel quality is superior. Therefore, it would be a waste of uplink resources to transmit a large feedback report which effectively only causes the network not to allocate resources on such component carrier. Particularly the transmission of subband MCSI or PMI costs a lot of bits. Regarding the data in Table 1 above, the sizes are valid for a single component carrier of 20 MHz bandwidth. Consequently, reporting detailed CQI for five such component carrier costs five times the number of bits as given in Table 1. Clearly this is not efficient in case that some component carriers have very low CQI.